Tactile stimulation of the skin excites cutaneous waves that travel tens of centimeters, but the implications for haptic engineering and perception are not well understood. We present evidence from optical vibrometry that tactile motion cues delivered via air-coupled ultrasound excite complex spatiotemporal wave fields in the hand. We distinguished two physical regimes based on the ratio of the motion speed to the cutaneous wave speed. At low speeds (1-4 m/s), waves generated by a moving stimulus propagated to similar distances in all directions. At high speeds (4-15 m/s), waves in the direction of motion were compressed. We also studied tactile motion perception at these speeds, which were faster than those used in prior studies. Motion sensitivity was impaired when waves were inhibited in front of the moving stimulus. This occurred for motion at high speeds and across disconnected skin areas. Together, our findings suggest that tactile motion perception is aided by waves propagating in the skin. This paper presents the first time-resolved observations of cutaneous responses to focused ultrasound, and contributes practical knowledge for the use of tactile motion and mid-air haptic feedback.

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Emerging holographic haptic interfaces focus ultrasound in air to enable their users to touch, feel, and manipulate three-dimensional virtual objects. However, current holographic haptic systems furnish tactile sensations that are diffuse and faint, with apparent spatial resolutions that are far coarser than would be theoretically predicted from acoustic focusing. Here, we show how the effective spatial resolution and dynamic range of holographic haptic displays are determined by ultrasound-driven elastic wave transport in soft tissues. Using time-resolved optical imaging and numerical simulations, we show that ultrasound-based holographic displays excite shear shock wave patterns in the skin. The spatial dimensions of these wave patterns can exceed nominal focal dimensions by more than an order of magnitude. Analyses of data from behavioral and vibrometry experiments indicate that shock formation diminishes perceptual acuity. For holographic haptic displays to attain their potential, techniques for circumventing shock wave artifacts, or for exploiting these phenomena, are needed.